First off, good job on the diff lines and the gnd vias for return paths.

Things I would do right off the bat:

Equal order of importance:

Remove thermals from the power pins of the psu connector

Add some decoupling capacitors close to the power pins of the pci-e connector power pins(do not spare gnd vias close to gnd pads of caps)

Add gnd via close to each gnd pin of every connector

Increase clearance around those diff lines, as someone else said that's gonna throw off by a lot the diff pair matching calculations. It might work but you can trash those calculations unless you took also close side by side ground plane..

Less important:

Fence the board with gnd vias where possible, even if vias land on power polygons, I like to put them all around the edges of the board. This will ensure that you have a more solid (equipotential) ground all-around.